This invention relates in general to a method of controlling engine idle speed in an automotive vehicle. More particularly, this invention relates to a method of controlling engine idle speed when the transmission""s forward clutch is engaged upon releasing the service brakes during launch from neutral idle operation.
Neutral idle operation of a vehicle can be initiated when a vehicle is brought to a stand still position with the engine still running, as when a vehicle is stopped at a traffic light. In such situations, the transmission can be disengaged, i.e., neutral idle operation, which is beneficial to decrease overall vehicle fuel consumption by unloading the engine.
More technically speaking, neutral idle operation of a vehicle is generally characterized by (i) the vehicle being at rest, (ii) the service brakes applied, (iii) the gear select lever in a forward range, and (iv) all combinations of torque transmitting clutches that establish a speed ratio from the input to the output member of the transmission being disengaged. As a result, during neutral idle operation, the transmission input shaft rotates freely at a substantially synchronous speed with the engine output shaft.
When a vehicle begins to move after being held stationary, it is said that the vehicle is launching from neutral idle operation. One method of launching from neutral idle operation is for the vehicle operator to merely cease applying the service brakes and allow the vehicle to creep forward. The inventors herein have recognized that this type of slow launch from neutral idle operation sometimes causes the vehicle to exhibit undesirable noise, vibration, and harshness (NVH) during the launch. The undesirable NVH is primarily a result of the engine speed changing during the launch from neutral idle to accommodate and react to the increased engine load of the transmission upon re-engagement. Specifically, as the transmission""s clutch pressure increases or decreases, the torque load on the vehicle""s torque converter turbine, and therefore also on the impeller, also increases or decreases. As a result, the transmission""s forward clutch begins to slowly engage to transfer the engine torque through the transmission, and the vehicle begins to slowly creep ahead. However, because conventional methods of engine speed control regulate the engine speed based on an assumption that the transmission is either in a xe2x80x9cDrivexe2x80x9d or xe2x80x9cNeutralxe2x80x9d mode of operation (as opposed to a partial mode in between xe2x80x9cDrivexe2x80x9d and xe2x80x9cNeutralxe2x80x9d), the engine speed control system tends to respond unevenly throughout the launch period. This is because the engine speed control system is reacting to the slowly increasing load on the engine.
Until recently, the NVH that results from the changing engine speed when the transmission engages has been relatively acceptable, primarily because vehicle operators have come to expect a certain amount of NVH when they physically move the vehicle""s gear shifter from xe2x80x9cNeutralxe2x80x9d (or xe2x80x9cParkxe2x80x9d) to a xe2x80x9cDrivexe2x80x9d position. However, because neutral idle operation is typically initiated by the vehicle itself, without the vehicle operator physically moving the gear shifter, the NVH that results from conventional engine speed control methods is more noticeable and undesirable.
The present invention relates to an improved method of controlling engine idle speed during launch from neutral idle operation. The present invention is particularly useful for minimizing undesirable noise, vibration, and harshness during a relatively slow launch from neutral idle operation, when the NVH is more noticeable to the vehicle operator. The present invention controls engine idle speed during launch by determining a xe2x80x9cfeedforward engine torquexe2x80x9d required to maintain a constant engine idle speed when launching from neutral idle operation. The feedforward engine torque is an estimated engine torque that is predicted to be required at a time in the future during the launch. The feedforward engine torque is calculated by the vehicle""s microprocessor based on an event that occurred in the drivetrain, such as the operator releasing the service brakes to begin engagement of the forward clutch. When the vehicle operator releases the service brakes, it is assumed that the vehicle is launching from neutral idle operation. The vehicle""s microprocessor then controls the engine speed based upon the calculated feedforward engine torque. As a result of the present invention, the increased torque demands from the re-engagement of the transmission are anticipated, and the engine speed can be controlled proactively instead of reactively, as in the prior art. The proactive engine speed control minimizes the NVH common in prior art systems.
The feedforward engine torque required to maintain a constant engine idle speed is determined by one of two preferred methods. In the first preferred method, the commanded or measured clutch pressure is used to estimate the torque capacity of the forward clutch at any given time throughout the launch period using a mathematical predictive model. Using this preferred method, the torque capacity of the forward clutch is estimated at various times based upon certain operating parameters, such as clutch pressure. In addition, a maximum torque converter load is determined based on the engine speed and other transmission variables, such as transmission fluid temperature, clutch design, and the like. The torque converter load when the clutch is fully engaged (turbine speed equal to zero) is imposed as an upper limit on the estimation of the maximum torque converter load. Then, a feedforward or predicted torque converter load is determined by comparing the maximum torque converter load and the torque capacity of the forward clutch.
In the second preferred method, the feedforward torque converter load is determined by a predetermined mathematical function. Specifically, the feedforward torque converter load at any given time can be determined purely as a function of time based on the engine speed, the turbine speed, and other transmission variables, such as transmission fluid temperature, clutch design, and the like. Using this second preferred method, a predicted turbine speed can be estimated based on its current rate of change. Either of the preferred methods will result in a reduction of NVH compared to prior art methods.